The CIA has been interrogating al-Qaida prisoners at a Soviet era compound in eastern Europe as part of a covert jail system set up after the September 11 attacks, according to the Washington Post. The secret facility is part of a network of "black sites" spanning eight countries, the existence and locations of which are known only to a handful of US officials and usually only the president and a few top intelligence officers in the host countries.

The internment network has also been kept almost entirely secret from the US Congress, which is charged with overseeing the CIA's covert actions, the newspaper said.

The CIA refused to comment on the allegations yesterday, but human rights groups demanded an urgent inquiry. "We've long been concerned that the USA could be running a totally secret network of 'war on terror' prisons and these claims need to be urgently investigated," an Amnesty International spokesman said.

Citing several former and current US intelligence and other officials, the Post said the CIA was holding the top 30 al-Qaida suspects at the secret facilities, where they were kept in dark, sometimes underground, cells in isolation from the outside world. They have no recognised legal rights, and no one outside the CIA is allowed to talk with them or see them.

The covert prison system was set up nearly four years ago in eight countries, including a facility in Thailand that was closed down after its existence was made public in 2003.

Concerns over the CIA's handling of prisoners escalated last week after it emerged that the US vice-president, Dick Cheney, and the agency's director, Porter Goss, asked Congress to exempt the agency from legislation banning the cruel and degrading treatment of prisoners. CIA agents have long been accused of playing a role in the prisoner abuse scandals at Abu Ghraib in Iraq and Guantánamo Bay on Cuba.

It is illegal for the American government to hold prisoners in such isolation in the US, which is why the CIA placed them overseas. But sources told the Post that the process has caused considerable internal debate within the agency, where there is concern about the legality, morality and practicality of the system.

While the Post declined to identify the countries in eastern Europe on security grounds, and governments in the region refused to comment, analysts pointed to the feverish competition among the east Europeans to host new US military bases.

The region's new Nato members, particularly Poland, Romania and Bulgaria, have been among Washington's staunchest allies in the campaigns in Iraq and Afghanistan, heightening speculation that they would be the likeliest venues for the secret jails. Romania and Bulgaria made military facilities available to the Americans for the Afghan and Iraq wars. The Pentagon is planning to dispatch 5,000 servicemen to a string of new bases in the two countries from next year.

In the run-up to the Iraq war the US also used a former Soviet airbase at Taszar in southern Hungary to train Iraqi exiles for police and military duties, although the scheme was quickly abandoned.

Legal experts and intelligence officials told the Post that the CIA's internment practices would also be illegal under the laws of several of the host countries, which have signed the UN convention against torture and other cruel, inhuman or degrading treatment and punishment.

The Post said it did not publish the names of the countries involved at the request of US officials, who claimed the disclosure might make them targets of terrorist retaliation.